No one was hurt from the twister that struck University City in a densely populated area of St. Louis County, touching down at 5:23 a.m. City officials said at least 100 homes were damaged. The tornado was apparently a small one -- meteorologists characterized it as between an EF0 and an EF1, meaning winds were likely between 65 mph and 110 mph.

National Weather Service meteorologist Jayson Gosselin said the tornado downed several power lines and trees, some of which blocked major roadways. The storm was also apparently to blame for a natural gas leak that forced the evacuation of some University City residents. The city opened a shelter at Heman Park for evacuees.

Gosselin said it was fortunate that the damage in University City wasn't worse, and that no one was hurt.

"There's a lot more to hit when a tornado is in an urban or suburban area, and a lot more people as well," he said.

Gov. Jay Nixon, citing the twister and the flash floods, declared a state of emergency and activated the State Emergency Operations Center.

"With the continued significant risk of severe storms and flooding, I urge Missourians to closely monitor weather conditions, so they can take shelter or move to higher ground if needed," Nixon said in a statement.

Storms knocked out power in suburban St. Louis and near Richwoods in a rural area about 50 miles southwest of St. Louis. The utility Ameren Corp. reported about 6,000 Missouri outages as of midmorning Thursday.

Rainfall was heavy over much of the state, especially in western Missouri. The National Weather Service said portions of Johnson County had more than 5 inches of rain, causing flash flooding that forced evacuation of some homes in the Warrensburg area. Several homes were damaged after several creeks flooded, said Sgt. Collin Stosberg of the Missouri State Highway Patrol.

The weather service said Highway T in Johnson County was closed after rushing water washed out three culverts. At least two motorists had to be rescued from flash flooding.

"Especially in the darkness people need to pay attention and not drive across high water," Stosberg said.

A three-person Johnson County emergency crew was briefly stranded when the jet skis they were using clogged with debris and broke down. The rescuers were unaccounted for early Thursday before being located downstream shortly after dawn.

Portions of mid-Missouri also had 3-4 inches of rain. Several roadways were flooded and closed.

Gosselin said the unsettled weather was expected to end by Friday. Temperatures will drop - highs over the next five to seven days will be mainly in the 50s, but conditions will be dry, Gosselin said.